An ice making machine is a particular version of a generally well known device which may be referred to as a refrigerator or heat pump depending upon the specific application of the device. Mechanical refrigeration is the process of absorbing heat from a substance and transferring this heat elsewhere--usually to the atmosphere--through a cooling medium. The most common cooling mediums are water or air. In mechanical refrigeration systems, the transfer of heat is accomplished through the use of commercial refrigerants which are capable of absorbing heat and boiling to gases at relatively low pressures and temperatures, and then giving up the heat as they condense into liquids at higher pressures and temperatures.
In its basic form a refrigeration system includes a compressor, a condenser, and an evaporator as its main elements. Most systems also usually include some sort of liquid control system, a reservoir referred to as a receiver, and suitable piping and valves.
The compressor is the device in the system that draws the cold, heat carrying refrigerant in the gas phase from the evaporator at relatively low pressure and temperature. The compressor raises both the pressure and temperature of the gas to the point at which the gas will condense to a liquid at ordinary water or air temperatures, typically between about 85.degree. F. and 105.degree. F.
The refrigerant typically flows from the compressor to the condenser. The condenser transfers the heat absorbed by the refrigerant in the evaporator to the atmosphere through the condenser's own cooling medium, which in common applications is water or air. In general, the refrigerant will condense from the gas phase to the liquid phase at this point.
The evaporator is the cooling component of the system in which the pressure is reduced and the liquid refrigerant allowed to boil to a gas at a relatively low temperature. This change of state absorbs heat from the substance surrounding the evaporator.
The liquid control system pipes and pumps the refrigerant from the evaporator, to the compressor, to the condenser, and back to the evaporator. Additionally, the system includes a liquid control device immediately ahead of the evaporator. This is typically an expansion or float valve which meters the proper amount of liquid refrigerant to the evaporator and which seals the high pressure and low pressure sides of the system from one another. The receiver stores a sufficient quantity of high pressure liquid refrigerant to insure a constant supply of liquid refrigerant to the liquid control device at all times.
A shell ice maker is a particular type of refrigeration system in which the evaporator takes the form of vertically oriented stainless steel tubes upon which water is sprayed and freezes into ice as the evaporator is cooled. U.S. Pat. Nos. 2,739,457 to Chapman and 4,324,109 and 4,404,810, both to Garland, are illustrative of shell ice making machines. Other background information on shell ice making machines is available from the Frick Division of York International Corporation of Waynesboro, Pa.
In a shell ice maker, water is sprayed onto the stainless steel tubes which make up the evaporator and freeze in place upon those tubes. A typical refrigerant for such a shell ice maker is ammonia or one of the appropriate chlorofluorocarbons. In order to harvest ice from the ice maker, however, some mechanism must be incorporated for removing ice from the stainless steel tubes. The most common method is to operate the entire ice maker on a timed cycle. In the major portion of the cycle, liquid refrigerant is pumped to the evaporator and allowed to evaporate, thereby cooling the evaporator and encouraging ice to freeze on the stainless steel tubes as water is sprayed upon them.
In the shorter portion of the cycle, and in order to remove the ice from the tubes, the ice maker also includes a series of pipes and valves for directing the warmer gas phase refrigerant into the stainless steel tubes of the evaporator. The warmer gas phase refrigerant in turn warms the tubes and melts at least a small portion of the ice on the exterior of the tubes so that the remainder will tend to slide off under the influence of gravity. When the ice strikes the lower portions of the ice maker, it breaks into pieces, and if necessary, is subjected to further mechanical breaking action to reduce the size of the pieces even further.
Such a typical shell ice making device will include the usual elements of the compressor and the condenser along with a common liquid-gas header for some or all of the stainless steel tubes of the evaporator, and a liquid-gas accumulator for regulating the flow of gas and liquid throughout the entire system. In a typical arrangement, the accumulator is physically located above the header so that liquid refrigerant can be added to the evaporator simply by allowing it to flow downwardly under the influence of gravity from the accumulator, into the header and then into the freezing tubes.
Refrigerant in the gas phase, however, will not automatically flow into the freezing tubes and must be specifically drawn out of the accumulator, compressed, and pumped into the freezing tubes. This pumping action, as would be expected, requires that a sufficient amount of mechanical energy be expended in order to draw the gas phase refrigerant from the evaporator, through the piping and liquid control system, into the accumulator, and finally into the compressor.
In typical ice making operations, an ice making cycle is selected that will produce a certain amount of ice and is represented by a certain time period during which liquid refrigerant must be circulated through the evaporator. Correspondingly, the harvesting cycle is likewise represented by a certain period of time during which gas phase refrigerant must be circulated through the evaporator to harvest the ice just grown. As an example, in a ten minute ice making cycle, liquid refrigerant would be circulated through the evaporator tubes for about seven and half minutes and then gas phase refrigerant would be cycled through for about two and half minutes. Thus, approximately twenty-five percent of every cycle is spent harvesting ice, not because it takes the ice that long to fall from the tubes, but because it takes that proportion of time to circulate enough of the gas phase refrigerant through the entire system to transfer enough heat to melt the ice sufficiently for it to fall.
As another disadvantage of this system, when pumping gas throughout the system, the suction pressure of the compressor is raised rather significantly, often to seventy pounds or higher before the ice defrosts and harvests. As might be expected, this uses significant amounts of energy. As known to those familiar with such systems, the suction required from the compressor also becomes greater when the ambient temperature is warmer.
Finally, the proportion of any given cycle used to harvest ice represents energy--and therefore mechanical and economic resources--used to pump gas phase refrigerant throughout the system. Alternatively, if the ice could be harvested more efficiently, that same energy could be used to form more ice, or the energy needed to form any given amount of ice could be correspondingly reduced.
There thus exists the need to reduce the energy consumption of the ice making cycle as well as to reduce the load on the mechanical equipment such as the condensers and the compressors, as well as a lessening of the proportionate time required to form and then harvest any particular amount of ice.